It has long been known to connect a dimmer between an AC utility power source and an incandescent lamp to control the light output of the incandescent lamp. The dimmer contains a TRIAC, and controls the RMS value in such a manner as to truncate a portion of the AC waveform of the AC utility power. In recent years, LED lamps using LEDs as light sources have been replacing incandescent lamps, and LED lamps that can be used with dimmers are also available in the market.
Patent document 1 discloses an LED lamp which detects an applied voltage waveform (phase switching), obtains a dimming factor from the detection result, and generates a current to be supplied to an LED based on the dimming factor. Patent document 1 describes that the LED is driven by a current controlled by pulse-width modulation (PWM) or constant current control. In particular, when driving the LED with a current controlled by constant current control, the advantage is that the LED is essentially unaffected by variations in utility power supply voltage, and does not produce perceivable flicker.